Process for operating a two-stroke internal combustion engine

ABSTRACT

When a two-stroke combustion engine (10) runs in the no-load or underload mode with intermittent fuel injection, besides the injection nozzles (12) being activated intermittently, the throttle (18) is opened more than is usual on transition into the no-load or underload mode and the fuel is simultaneously injected at a constant angle of aperture and flow. The process ensures reliable ignition of a weak mixture in the no-load or coasting mode; thus the energy consumption and exhaust production are limited in these modes and the engine runs in a cost-effective manner.

The invention relates to a process for operating a two-stroke internalcombustion engine with an electronically controlled direct injection ofthe fuel into the cylinder, wherein the fuel injection is performedintermittently in the no-load and in the lower underload mode.

For two-stroke Otto engines it is known that by forming a mixture withinthe cylinder, i.e. by a fuel injection after the closing of the exhaustchannels, the consumption values and the HC proportions in the exhaustgas can substantially be improved. By such a direct injection of thefuel a layering of the loading in the interior of the cylinder can beobtained in that, namely, the fuel is injected by means of the injectionnozzle into defined areas of the cylinder and thus, in the moment ofignition, is in the region of the spark. However, it has been found thatin the lower underload mode, in the thrust mode as well as in theno-load mode there are still ignition irregularities, which in thesemodes are assumed to result from the high proportion of old gas in thecylinder. It is assumed that the reason for this is that the fuel,although being injected into the correct area, does not mix sufficientlyup to the moment of ignition within the large amount of old gas to forman explosive mixture with the fresh air introduced in relatively smallamounts due to the throttle being closed in the underload, thrust andno-load modes. Only after several gas changes have occurred, the old gasis dislodged by fresh air such that the injected fuel can burn.

In DE-PS 913 835 this phenomenon of the no longer regular ignition isdiscussed and it is assumed that the cylinders of the two-stroke Ottoengine are supplied with a too weak mixture and that therefrom theignition difficulties result. For removing these difficulties it issuggested at the change-over from full load mode to underload mode toproceed from a control with diminishing gas pressure with a constantopening time of the injection valves to a control with increasedconstant gas pressure with reduced opening times of the injectionvalves. Thereby, the fuel supply to the cylinders is performedintermittently, and this only at each second third, fourth or fifthrevolution or cycle. By this intermittent fuel supply the capability ofthe mixture to ignite is improved by the twofold repeated supply offresh air between the injection intervals. These improvements by theintermittent operation are only possible by a comparatively highexpenditure, because two control pumps are required, one of which isprovided with a sloping edge control. The described design permits onlyan injection at each second cycle; it must be questioned whether therebyalready a reliable ignition can be achieved.

From the journal "Kraftfahrzeugtechnik", Berlin 35 (1985), volume 7,pages 201 to 203, there is also known a two-stroke Otto engine withintermittent injection in the underload mode. By means of controlelectronics the amount of fuel as well as the moment in time aredetermined at which this amount of fuel is injected into the cylinder.For this a special pump-nozzle-system is being used. Thereby, theconsumption and the exhaust gas values shall be improved.

The invention, thus, is based on the problem to provide a two-strokecombustion engine in which--particularly in the underload mode as wellas in the no-load and the thrust modes--good circulation or runningproperties and a low underload consumption are guaranteed as well asincreased requirements for low exhaust gas values can safely beobtained.

The inventive solution of this problem is to be seen in that there isprovided a process for operating a two-stroke combustion engine with anelectronically controlled direct fuel injection into the cylinder,wherein the fuel injection in the no-load and the lower underload modeis performed intermittently, characterized in that the electroniccontrol for the intermittent operation of the injection nozzleadditionally opens the throttle beyond the no-load or underloadcondition in order to supply a larger amount of fresh air, and the fuelis injected during a load-depending but constant rotational angle with aconstant fuel volume flow.

By the features of the invention it is achieved in advantageous mannerthat by opening the throttle beyond the position common for the no-loadand the underload condition, respectively, a larger amount of fresh airis supplied, which insures the ignition of smallest amounts of fuelinjected during the intermittent mode, whereby a lean combustion with aperfect circulating operation is guaranteed, because the ignitionconditions for one performed injection are already insured afterrelatively few revolutions without injection. Since the injection ismade at a constant rotational angle or angle of aperture with a constantvolume flow, in the no-load mode a drifting-off of the number ofrevolutions is balanced, because a change of the number of revolutionsis automatically associated with an opposite change of the injected fuelvolume. Further, spark failures due to a wrong layering of the mixturein the cylinder are safely avoided.

It is also proposed that the throttle is completely opened during theno-load and underload conditions, respectively, in order to obtain alayering of the loading which in the moment of ignition brings theinflammable mixture near to the spark and, depending on the loadcondition, keeps more or less proportions of the pure fresh air inregions remote from the spark. In spite of the full opening of thethrottle, a regulation of the number of revolutions is not necessary inthe no-load mode, since by the injection of the fuel with constantvolume flow during a constant angle of aperture a drifting-off of thenumber of revolutions is automatically regulated.

However, in order to make possible a capacity regulation also in theunderload mode, it is further suggested that for adjusting a desiredengine capacity the rotational angle of aperture or injection isaltered, but then kept constant irrespective of the number ofrevolutions of the engine.

In order to make certain the injection with a constant volume flow inall operating conditions, the inventive method proposes that the fuel beinjected with a pressure of about 30 bar or more directly into thecylinder. By this increased injection pressure the atomization isimproved and thus, the formation of the mixture is made possible withina shorter time period.

Preferably, the duration of each injection is in the order ofapproximately 1 ms. Such control can be realized very exactly, becauseexactly working injection nozzles are available in the prior art, andthe adjustment of the aperture angle can easily be performed by means ofangle indicators arranged on the crankshaft.

In multiple-cylinder two-stroke engines the fuel injection for theindividual cylinders can be performed sequentially and periodically inequal time intervals, so that all cylinders can be switched off or canignite in series. In a four-cylinder radial type engine with a cylinderangle of 90° and an ignition distance of 90° (cylinder sequence 1, 2, 3,4) one can, for example, let the injection sequence rotate opposite tothe ignition sequence. Starting from cylinder no. 1, thus, fuel isinjected after 3/4 motor revolutions only into the cylinder no. 4, after11/2 revolutions into the cylinder no. 3, after 21/4 revolutions intothe cylinder no. 2 and after 3 revolutions again into the cylinderno. 1. Hereby, the fuel is injected only at each third revolution intoone of the four cylinders one after the other. Thereby, a uniformignition sequence in equal time intervals and a uniform rotation orcirculation result.

The invention with its advantages and features will be explained also inthe following description of an embodiment in connection with the claimsand the drawing.

FIG. 1 shows a diagram for a normal and intermittent direct injection ina two-stroke engine;

FIG. 2 shows a schematic representation of a circuitry for performingthe method according to the invention.

In the diagram according to FIG. 1, in the lower portion the ignitionsof the spark performed at each revolution of the engine and at thecorresponding rotational angular positions (θ) are illustrated. In theupper part of the diagram the injections are indicated, wherein in thefirst line the normal injection prior to each ignition, i.e. at eachrevolution of the engine is shown.

In the second and third lines of the diagram the periodicallyinterrupted injections are illustrated. In the course of the injectionillustrated in the second line a periodically intermittent fuelinjection is performed only at each second engine revolution or cycle.With the example illustrated in the third line a periodicallyintermittent fuel injection is performed only with each third enginerevolution or cycle. In a corresponding manner a fuel injection could beperformed only with each fourth, fifth etc. engine revolution or cycle.

In the illustration according to FIG. 2 there is schematically shown atwo-stroke combustion engine 10. The spark 11 ignites at each revolutionof the engine. At the cylinder head an injection nozzle 12 is providedwhich injects the fuel periodically into the cylinder space in equaltime intervals intermittently as desired at each individual, eachsecond, each third, each fourth etc. revolution. This injection isperformed shortly after the closing of the intake channel 13 and theexhaust channel 14, and shortly before the piston 15 reaches the upperdead center as well as prior to the moment of ignition. In the fresh airconduit leading to the intake channel 13 a throttle member with athrottle valve 18 is provided. This throttle member is connected with anelectronic control 20 which is fed with a control signal serving forcontrolling the engine during operation. The electronic control isconnected with an angle indicator 21 for the capacity regulation, whichinturn acts onto a valve 22 for the fuel injection. This valve 22 isdirectly connected with the injection nozzle 12 and supplies theinjection nozzle with the fuel in a constant volume flow under highpressure, for example with a pressure of 30 bar or more.

The angle indicator for the capacity regulation 21 comprises one or moresensors 25 associated with the crankshaft 26 of the engine and detectinga pre-determined rotational angle α per each revolution. During thisrotational angle the valve 22 is activated and opened for the fuelsupply towards the injection nozzle 12. Since the fuel is supplied witha constant volume flow, this releases with a constant number ofrevolutions during the rotational angle α (also named opening angle α inthe following) the injection of a constant fuel volume. If the number ofrevolutions changes--particularly during the no-load or the thrustmode--, i.e. if the number of revolutions drifts off, also the timeperiod will change during which the rotational angle α is passed, andthis occurs in a manner oppositely proportional to the number ofrevolutions. Thereby a self-regulation for the number of revolutionsresults in that, if the number of revolutions increases the amount ofinjected fuel decreases or when the number of revolutions decreases thetime period of fuel injection increases and thus, the amount of fuelinjected with a constant volume flow increases.

In order to guarantee according to the invention a safe ignition with amixture as weak or lean as possible, simultaneously with the change-overinto the no-load or underload mode the throttle valve 18 is opened bymeans of the electronic control 20. This results in a plentiful supplyof fresh air during the intermittent fuel injection. By the largeramount of supplied fresh air a safe ignition is guaranteed.Additionally, however, the injection can be made also more often insmaller amounts, which in spite of the intermittent injection has theresult that the engine obtains a very smooth circulating running.

In order to reduce the amount to be injected it is, therefore, proposedthat the sensors 25 by which the rotational angle or angle of aperture αis detected, permit an angular adjustment. This adjustment of the anglecan also be used for a regulation of the capacity or output in theunderload mode with opened throttle valve. By the plentiful offering offresh air it is guaranteed that a layering of the charge is obtained atwhich always an inflammable mixture collects near the spark at themoment of ignition and, according to the load condition, also a more orless large proportion of fresh air in the region remote from the spark.By this measure a regulation of the load condition by the injectionamount alone becomes possible also in the no-load and underload modes,whereby always plenty of fresh air is present in order to insure in theunderload mode a lean combustion, wherefrom the especially favourableexhaust gas values and consumption values result for a two-strokecombustion engine operated according to the method of the invention.

I claim:
 1. Process for operating a two-stroke combustion engine with anelectronically controlled direct fuel injection into the cylinder,wherein the fuel injection in the no-load and the lower underload modeis performed intermittently, characterized in thatthe electronic controlfor the intermittent operation of the injection nozzle additionallyopens the throttle beyond the no-load or underload condition in order tosupply a larger amount of fresh air, and the fuel is injected during aload-depending but constant rotational angle with a constant fuel volumeflow.
 2. Process according to claim 1, characterized in thatthe throttleis completely opened in the no-load or the underload mode.
 3. Processaccording to claim 1, characterized in thatthe injection of the fuelduring a constant rotational angle is controlled by an angle indicatorresponsive to the crankshaft position.
 4. Process according to claim 1,characterized in thatfor adjusting a desired engine capacity therotational angle is changed and then kept constant irrespective of thenumber of revolutions of said engine.
 5. Process according to claim 1,characterized in thatthe injection pressure amounts to at leastapproximately 30 bar.
 6. Process according to claim 1, characterized inthatthe electronically controlled fuel injection is performed with aduration of each injection in the order of 1 ms.
 7. Process according toclaim 1, characterized in thatin a two-stroke combustion engine havingmultiple cylinders the fuel injection for the individual cylinders isperformed sequentially and periodically in equal time intervals.